Organic thin films continue to attract great interest in the materials science community due to their ease of processing, ease of functionalization, light weight and flexibility. Significant progress has been achieved in the past 10–20 years, presenting the possibility of molecular level control in molecular and macromolecular composite films. The ionic, layer-by-layer assembly technique, introduced by Decher in 1991, is among the most exciting recent developments in this area. Decher, G.; Hong, J.-D. Makromol. Chem., Macromol. Symp. 1991, 46, 321–327; Decher, G.; Hong, J.-D. Ber. Bunsenges. Phys. Chem. 1991, 95, 1430–1434; and Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/211, 831. This approach, which utilizes electrostatic interactions between oppositely charged polyion species to create alternating layers of sequentially adsorbed polyions, provides a simple and elegant means of depositing layer-by-layer sub-nanometer-thick polymer films onto a surface using aqueous solutions. Lvov, Y. M.; Decher, G. Crystallography Reports 1994, 39, 628–647; Ferreira, M.; Rubner, M. F. Macromol. 1995, 28, 7107–7114; and Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171–2176. More recently, applications have been extended to electroluminescent LEDs, conducting polymer composites as well as the assembly of proteins and metal nanoparticle systems. Tian, J.; Wu, C. C.; Thompson, M. E.; Sturm, J. C.; Register, R. A.; Marsella, M. J.; Swager, T. M. Adv. Mater. 1995, 7, 395; Baur, J. W.; Kim, S.; Balanda, P. B.; Reynolds, J. R.; Rubner, M. F. Advanced Materials 1998, 10, 1452–1455; Cheung, J. H.; Fou, A. F.; Rubner, M. F. Thin Solid Films 1994, 244, 985–989; Ferreira, M.; Cheung, J. H.; Rubner, M. F. Thin Solid Films 1994, 244, 806–809; Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117–6123; and Ariga, K.; Lvov, Y.; Onda, M.; Ichinose, I.; Kunitake, T. Chemistry Letters 1997, 125–126.
Application of organic thin films to integrated optics, microelectronic devices, sensors and optical memory devices requires a means of patterning and controlling the surface architecture. Photolithography is the conventional patterning technique of choice, but lithographic techniques require materials designed to exhibit efficient responses to irradiation with a chemical change, namely crosslinking or degradation; these requirements are not trivial. Finally, light-based lithography can be limited in its application to curved, nonplanar surfaces, such as optical lenses and fibers, and multiple processing steps are required to create three dimensional, multiple level microstructures.
Patterning polymeric thin films in situ through the use of chemically patterned surfaces as templates for ionic multilayer assembly has been presented. Hammond, P. T.; whitesides, G. M. Macromolecules 1995, 28, 7569; Clark, S. L.; Montague, M.; Hammond, P. T. Supramol. Sci. 1997, 4, 141–146; Clark, S. L.; Montague, M. F.; Hammond, P. T. Macromol. 1997, 30, 7237–7244; Clark, S. L.; Hammond, P. T. Adv. Mat. 1998, 10, 1515–1519; Clark, S. L.; Handy, E. S.; Rubner, M. F.; Hammond, P. T. ACS Polym. Prepr. 1998, 39, 1079–1080; Clark, S. L.; Montague, M. F.; Hammond, P. T. ACS Symp. Ser. 1998, 695, 206–219; and Clark, S. L.; Handy, E. S.; Rubner, M. F.; Hammond, P. T. Advanced Materials 1999, 11, 1031–1035. Selective deposition was achieved by introducing alternating regions of two different chemical functionalities on a surface: one which promotes adsorption; and a second which effectively resists adsorption of polyions on the surface. More recent explorations have illustrated that by adjusting the ionic strength, pH and polyion chemical structure, one can tune the interactions between polyions and the surface functional groups, allowing different polyion pairs to be adsorbed on specific regions of the surface based on electrostatic, hydrogen bonding, and hydrophobic interactions.
One class of thin films is polyelectrolyte multilayers. These relatively inexpensive materials can be used electro-optical, conducting, and luminescent applications; recent developments include new functionalities such as electrochromic thin films, photovoltaics, ionically conducting systems, and even new biologically functional systems for cell templating and drug delivery. For each of these interesting functionalities, new applications in the areas of electro-optic and electronic devices, flexible displays, micropower and sensor applications, it is critical to be able to control the two- and three-dimensional placement of these films on substrates. Methods of patterning polyelectrolyte multilayers in-situ during the adsorption process utilizing chemically patterned surfaces that guide polymer adsorption to specific regions based on selective deposition have been reported. One important advantage to this approach is the fact that it is non-lithographic, thus allowing the creation of low-cost functional devices. Some challenges with this method include the need to tune the selectivity of deposition via processing conditions and polyion composition to achieve high selectivity. More recent approaches have been developed for patterning multilayers, including ink-jet printing via a subtractive mechanism of “erasing” multilayer films, and a range of photolithographic approaches. Many of these methods require the design a photolytic component either within the film or in the underlying layers, and therefore limit the range of polymer films used in the approach, as well as increasing the cost of manufacturing. Hence, a highly selective method for adhering a wide variety of polyelectrolyte films to a substrate would alleviate some of the current limitations.
The fabrication of polyelectrolyte multilayer thin films has received much attention recently as a simple yet versatile technique for assembling various thin film optoelectronic devices and nanostructured thin film coatings (for a review, see: Decher, G. Science 1997, 277, 1232). Since the layer-by-layer process creates nanostructured-controlled polyelectrolyte complexes, which have already exhibited a long research history as biomaterials, several groups have begun to realize the potential of multilayers for biomedical applications, including biosensor and cell encapsulation applications. Michaels, A. S. Ind. Eng. Chem. 1965, 57, 32; Decher, G.; Lehr, B.; Lowack, K.; Lvov, Y.; Schmitt, J. Biosens. Bioelectron. 1994, 9, 677; Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3427; Schneider, S.; Feilen, P. J.; Slotty, V.; Kampfner, D.; Preuss, S.; Berger, S.; Beyer, J.; Pommersheim, R. Biomaterials 2001, 22, 1961. Recently, some groups have investigated more specifically the interactions of multilayers with living cells. Chluba, J.; Voegel, J.-C.; Decher, G.; Erbacher, P.; Schaaf, P.; Ogier, J. Biomacromolecules 2001, 2, 800; Grant, G. G. S.; Koktysh, D. S.; Yun, B.; Matts, R. L.; Kotov, N. A. Biomed. Microdevices 2001, 3, 301; Serizawa, T.; Yamaguchi, M.; Matsuyama, T.; Akashi, M. Biomacromolecules 2000, 1, 306; Elbert, D. L.; Herbert, C. B.; Hubbell, J. A. Langmuir 1999, 15, 5355; Tryoen-Tóth, P.; Vautier, D.; Haikel, Y.; Voegel, J.-C.; Schaaf, P.; Chluba, J.; Ogier, J. J. Biomed. Mater. Res. 2002, 60, 657. For instance, it has been shown that melanoma cells could sense and respond to signaling hormone molecules immobilized within polylysine/polyglutamic acid multilayers, that muscle and neuronal precursor cells readily attached to collagen/sulfonated polystyrene (SPS) multilayers, and that, depending on whether chitosan or dextran sulfate was the outermost layer, multilayers assembled from those biopolymers alternately showed either pro- or anticoagulant properties, respectively, with human blood. Chluba, J.; Voegel, J.-C.; Decher, G.; Erbacher, P.; Schaaf, P.; Ogier, J. Biomacromolecules 2001, 2, 800; Grant, G. G. S.; Koktysh, D. S.; Yun, B.; Matts, R. L.; Kotov, N. A. Biomed. Microdevices 2001, 3, 301; Serizawa, T.; Yamaguchi, M.; Matsuyama, T.; Akashi, M. Biomacromolecules 2000, 1, 306. In addition, alginate/polylysine multilayers, when deposited onto otherwise cell-adhesive substrates, such as extracellular matrix (ECM), could render those surfaces cell resistant. Elbert, D. L.; Herbert, C. B.; Hubbell, J. A. Langmuir 1999, 15, 5355. The effect of the outermost surface layer of various multilayer systems on the in vitro response of osteoblasts has recently been investigated, as well. Tryoen-Tóth, P.; Vautier, D.; Haikel, Y.; Voegel, J.-C.; Schaaf, P.; Chluba, J.; Ogier, J. J. Biomed. Mater. Res. 2002, 60, 657.
A novel and desirable advancement in this art would be a method to stamp a polyelectrolyte microlayer directly onto other surfaces, particularly plastic substrates and multilayer films, by careful selection of surface chemistry. The motivation for establishing these routes are two-fold. First, the functionalization and subsequent patterning of ionic multilayers and other materials on a broad range of substrates, including polymeric surfaces, without elaborate pretreatment would extend the scope of the application. Second, stamping atop continuous or patterned polymer thin films, followed by subsequent selective adsorption or deposition steps would generate complex multiple level heterostructures. The ability to create multi-level microstructures with layer-by-layer assembly enables the construction of devices such as transistors, diodes, sensors, and other optical and electrical components. The chemical patterning of the top surfaces of multilayer films also brings new opportunities to incorporate other materials onto multilayer films; the use of such chemical patterns to direct materials deposition can lead to the patterning of metal electrodes, the placement of colloidal particles, or the directed deposition of other polymer films atop layer-by-layer functional thin films. Importantly, the ability to create patterned functional chemistry atop a polyelectrolyte surface would enable modification of any surface which can be covered with at least one surface layer of polyion.